In varactor tuners, which are often referred to as electronic tuners, varactor diodes exhibit capacitance variations with changes in bias voltage and serve as the variable reactances in-otherwise-conventional tuned circuits. Such tuning systems are easy to tune, free from RF signal carrying contacts and afford the designer great versatility in receiver styling. As pointed out in the related applications above, their most serious drawbacks are the limited range of diode capacitance change and the nonlinear relationship between frequency and bias voltage. The Federal Communications Commission so-called "equal tuning" rule for VHF and UHF television channels added difficulty to an already formidable problem.
The invention in the first-mentioned related application provides an attractive solution to these problems. In brief, that system produces a separate "slope factor" which is related to the slope of the tuning voltage-versus-frequency characteristic for proportioning the "fine" tuning voltage such that equal frequency excursions are experienced for equal tuning information increments. The result is a truly "equalized" tuning system. In the environment of the above invention the slope factors are stored in appropriate memories as are the nominal (coarse) tuning informations and fine tuning informations. The memories are channel number addressed. For each channel selection a nominal tuning voltage information, a fine tuning voltage information and a slope factor are produced. The fine tuning information is multiplied by the slope factor and combined with the nominal tuning information for conversion to the final tuning voltage.
The second-mentioned referent application is particularly concerned with memory utilization in digital tuning systems and the savings in memory which may be achieved by use of the slope factor. With the exception of four discontinuities between the low VHF (channels 2-4), mid VHF (5, 6), high VHF (7-13) and UHF (14-83), adjacent channel numbers represent 6MHz increments in frequency. Memory saving is based upon storing reference or initial value tuning information for a reference or pseudo channel in each frequency band and separate tuning increment information, representing the tuning voltage changes required to successively tune from one channel to the next, beginning with the reference or pseudo channel. These increments are the difference equation analog of the slope factors. Upon channel changes, an arithmetic computation is performed in which the initial value information and successive increment informations are added. The initial value tuning information is selected at a point 6MHz below the lowest numbered channel in the band which point is then referred to as the pseudo channel number. Thus, in the low VHF band, for instance, rather than storing complete information words corresponding to the nominal tuning information for channels 2-4, the nominal tuning information for pseudo channel 1 is stored along with the slope factors or increments required to go from pseudo channel 1 to real channel 2, from channel 2 to channel 3, and from channel 3 to channel 4. Suitable logic and apparatus are provided for summing the pseudo channel information and successive increments for obtaining the nominal tuning information corresponding to the selected channel number.
The last increment represents the slope factor of the tuning curve at the selected channel, and is separately available. It is therefore readily usable for equalization of any auxiliary tuning voltage source to provide true equalized tuning for the system.
The present invention uses almost identical structure as that of the second referent application, but does not derive the nominal tuning voltage from a reference channel nominal tuning information and individual tuning increment information. Rather the tuning information increment or slope factor is derived (for equalization purposes) from the stored nominal tuning information for each channel. Reference channel tuning information is required for deriving the slope factor for an end channel in the band.
The present system is extremely attractive in that the slope factors are readily obtainable without resort to any special equipment. As discussed in the other applications, either a zero or fixed offset fine tuning system may be used. In the fixed offset system of the preferred embodiment, the nominal tuning information is selected to be 1/2 the equalized fine tuning range information below the tuning curve.
The art teaches memory storage of channel tuning information at addressable locations as exemplified by U.S. Pat. No. 3,990,027. There is no known art, other than that represented by the referent copending applications of the invention, relative to slope factor derivation.